Obligatory Charity Tax (Zakat)

Bukhari :: Book 2 :: Volume 24 :: Hadith 536

Narrated Abu Huraira:

Abu Bakr said, "By Allah! If they (pay me the Zakat and)
with-hold even a she-kid which they used to pay during the
life-time of Allah's Apostle, I will fight with them for it." 'Umar
said, "It was nothing but Allah Who opened Abu Bakr's chest towards
the decision to fight, and I came to know that his decision was
right."